Reinforcing fillers such as carbon black and silica have long been used for reinforcing rubber as is well known. In the rubber industry, such as the tire industry, the practice has been to mix the carbon black used for rubber reinforcement with the rubber in advance using the wet carbon black master batch method so as to simplify mixing step of the carbon black with the rubber and to improve the dispersion of the carbon black in the rubber (for example, see Japanese Unexamined Patent Publication (Kokai) No. 59-49247 and Japanese Unexamined Patent Publication (Kokai) No. 63-43937). However, in recent years, the development of superior silane coupling agents has led to formulation of silica instead of carbon black. Since silica, as compared with carbon black, has properties of providing a low tan δ at high temperatures (i.e., around 60° C.) and a high tan δ at low temperatures (i.e., around 0° C.), when used for rubber compositions for tire treads for example, a tire having a low rolling resistance and a high gripping power can be advantageously produced. However, silica is inferior to carbon black in the abrasion resistance and the low electrical conductivity, and therefore, if used for tires, there is the problem that the tire will pick up a charge when driven on and will cause various problems such as noise and, in some cases, even misoperation in electronic equipments such as radios.
Covering the surface of a pigment, etc. with silica etc. to improve the dispersion and to increase weather resistance has been proposed in, for example, Japanese Examined Patent Publication (Kokoku) No. 50-14254 and Japanese Examined Patent Publication (Kokoku) No. 7-30269. For example, Japanese Examined Patent Publication (Kokoku) No. 7-30269 discloses a method of treating the surface of carbon black used for a powder paint comprising the steps of dispersing carbon black in water, adjusting the pH to 6 or more, and, while maintaining the temperature at least at 70° C., precipitating amorphous silica on the surface of the particles of carbon black using sodium silicate. However, none of these publications discloses the deposition of silica on the surface of the carbon black used for reinforcing rubber. Further, Japanese Unexamined Patent Publication (Kokai) No. 8-277347 discloses the adhesion of silica to the surface of carbon black used for rubber reinforcement, but the industrially efficient production process is not disclosed at all.
When formulating silica into a rubber composition, the silica is difficult to disperse during the mixing, and therefore, a large amount of labor work is required in the mixing process, and therefore, production of master batches as in the case of carbon black is desired. However, since silica has a coagulating pH (i.e., about 4 to 7) different from the coagulating pH region (i.e., about 2.5 to 3) of rubber latex and for other reasons, a wet silica master batch has not been satisfactorily produced.